This invention relates generally to acoustic wave devices, and, more particularly, to surface acoustic wave bandpass filters for use in a variety of applications, particularly in the communications field.
Surface acoustic wave (SAW) devices are a relatively new class of devices that utilize the propagation of surface acoustic waves in various piezoelectric materials, at ultrasonic frequencies. In recent years, such devices have been advanced to an extremely practical stage of development, and they are now widely used in communications and signal processing systems. The underlying reason that ultrasonic waves provide the basis of a useful class of filters, is that such waves travel with negligible loss in suitable solids, at typical velocities of 10.sub.3 to 10.sub.4 meters per second. These velocities correspond to operating frequencies in the range 30 to 800 megahert (MHz) for practical surface wave filters.
When an acoustic wave is propagated along the surface of a piezoelectric material, a traveling electric field is also generated on the surface. This field also extends significantly above the surface, and can interact with appropriately constructed metal electrodes disposed on the surface. Typically, such electrode structures are formed by photolithographic techniques, and take the form of interdigital transducers. An interdigital transducer is a two-terminal device comprisng a plurality of parallel metal strips or fingers, uniformly spaced on the surface of the piezoelectric substrate. The fingers are connected in alternating fashion to two terminals strips, having the general appearance of two combs with their teeth interleaved but not touching. When a voltage is applied across the terminals of an interdigital transducer, electric fields are generated within the substrate, and these generate corresponding stress patterns as a result of the well known piezoelectric effect. If the voltage applied to the terminals is an alternating signal of suitable frequency, the value of which is dictated by the transducer finger spacings, elastic surface waves are launched in two opposite directions, perpendicular to the transducer strips. The transducer is then said to be functioning as a transmitting transducer, or an input transducer. A similarly structured receiving or output transducer can convert the traveling surface waves propagated from the transmitting transducer back into an alternating electrical signal.
Since an interdigital transducer will operate only at frequencies within a relatively narrow range, determined by the transducer geometry, a surface acoustic wave device comprising a pair of such transducers provides a highly effective bandpass filter. Further background information on surface wave filters and related devices can be obtained in any of the large number of publications. For example, a book entitled "Surface Wave Filters, Design, Construction, and Use", edited by Herbert Mathews, and published by John Wiley and Sons, New York (1977), contains a good deal of useful background material on surface acoustic wave devices, as well as a comprehensive bibliography on the subject.
The frequency response characteristics of an interdigital transducer are essentially those of a bandpass filter having a relatively low insertion loss over its pass band. Consequently, a good bandpass filter can be constructed utilizing a pair of interdigital transducers mounted on the surface of a piezoelectric substrate, and the characteristics of such a filter will be determined largely by the geometry of the transducers. More specifically, the finger spacing of the transducers determines the mid-band frequency of the filter, and the number of fingers determines the bandwidth. The time delay from input to output of the filter is, of course, determined by the spacing between the input and output transducers.
A well known source of distortion in surface acoustic wave filters is due to a phenomenon referred to as the triple transit effect, or triple transit echo. Part of the signal received by the output transducer from the input transducer will be reflected back toward the input transducer. The magnitude of the reflected wave will depend in part on the degree of matching between the characteristic impedance of the receiving transducer and the impedance of the electrical load circuit to which it is connected. When the transducer is optimally matched with its load, i.e., matched for maximum power transfer, the magnitude of the reflected wave is at a maximum. Part of the reflected signal will be again reflected back from the input transducer toward the output transducer. Accordingly, the principal or intended output signal from the output transducer will be distorted by a triple transit echo signal that is delayed in time by twice the transit time of the principal signal. The triple transit echo results in significant ripples on both the amplitude characteristic and the time delay characteristic of the filter, both plotted with respect to frequency.
The most straightforward technique to reduce the triple transit effect is to mismatch the transducer's load impedance, and thereby increase the insertion loss of the filter. For every one decibel (dB) increase in the insertion loss of the filter, there is a 2 dB reduction in the triple transit echo signal. Naturally, this added insertion loss is usually viewed as a disadvantage in filter design, and other techniques for reduction of the triple transit effect have been sought.
One known technique for reducing the triple transit effect is to add a third transducer spaced from the input transducer by a distance one-quarter wavelength greater than the distance to the normal output transducer. The resultant waves reflected from the normal output transducer and the third transducer tend to cancel and thereby reduce the triple transit effect. However, this solution is not easy to implement at higher frequencies, because manufacturing tolerances make it difficult to obtain an extremely accurate one-quarter wavelength spacing. Moreover, such a device cannot be tuned to operate over a range of frequencies.
It will be appreciated from the foregoing that, prior to this invention, there has been a need for a convenient and reliable technique for minimizing the effect of triple transit echo signals in surface acoustic wave filters. The present invention fulfills this need.